Ecosystem restoration and, in particular, peatland restoration, are considered a promising greenhouse gas (GHG) mitigation strategy to move towards net zero emissions. To remain within acceptable limits for projected warming scenarios, inaction with respect to GHG mitigation in the short term implies a need for even larger removals of GHGs in the longer term, which can be conceptualized as a 'mitigation debt'. This paper explores the economic implications of delaying GHG mitigation through ecosystem restoration using data of a large survey (N=1,377) that included a choice experiment to elicit the public's willingness to pay (WTP) for peatland restoration in Scotland, UK. The valuation specifically considers the interaction between the timing of restoration action and long-term ecosystem resilience. We find that respondents have a substantial WTP for peatland restoration. Importantly, we find considerable benefits for early restoration action, which is linked to an increased resilience of peatlands under future climate change. This demonstrates that delaying restoration and thus accumulating a mitigation debt has an important opportunity cost that substantially decreases the related economic benefits. Attitudes towards climate change and climate change beliefs are found to explain variation in the public's WTP. Our research strengthens the economic argument for not delaying climate change mitigation through ecosystem restoration, demonstrating that the mitigation debt also translates into a welfare loss. To fully realise the potential benefits associated with immediate mitigation using peatland restoration, however, more needs to be understood about the mechanisms that facilitate large-scale implementation in practice.